1. Field of the Invention
This invention relates to a lubricating oil composition for internal combustion engines having improved fuel economy, wear resistance and antioxidancy properties.
2. Description of the Related Art
While the majority of moving parts in an internal combustion engine are in a state of hydrodynamic lubrication, some sliding parts such as pistons and valve trains are in a boundary lubrication state. In order to provide wear resistance caused by friction in the boundary lubrication state, it is necessary to provide the engine oil with additives to reduce wear. For many years, zinc dialkyldithiophosphates ("ZDDP") have been a standard antiwear additive. While ZDDP is a good antiwear agent, it has negative impacts on fuel economy. Thus it is usually necessary to include a friction modifier for fuel economy purposes. Both antiwear and friction modifiers function through adsorption on the sliding metal surface and may interfere with each other's respective functions.
U.S. Pat. No. 4,705,641 describes an engine oil having improved antiwear and antioxidancy properties. The engine oil contains from 0.002 to 0.3 wt % of a copper salt and from 0.004 to 0.3 wt % of a molybdenum salt. This combination is also stated to reduce the treat rate of ZDDP necessary for wear protection. There is no mention of the frictional properties of the engine oil.
European patent application EP 562,172 describes an engine oil having improved frictional properties. The engine oil contains a boron compound derivative of alkenylsuccinimide, an alkaline earth metal salt of salicylic acid and either or both of a molybdenum dithiophosphate and a molybdenum dithiocarbamate. There is no mention of the wear or antioxident properties of the claimed additive combination.
It is well known in the art that in formulating engine oils, there is a delicate balance between friction and wear performance. It would be desirable to have an engine oil with improved fuel economy, wear resistance and antioxidancy to meet the increasing performance demands placed on modern oils due to environmental considerations.